function factorial(num){
  if(num <= 1){
    return 1
  }
  return num * factorial(num-1);
}
console.log(factorial(6));//720

let trueFactorial = factorial;
factorial = function(){return 0}
console.log(trueFactorial(6));

function factorial1(num){
  if(num <= 1){
    return 1;
  }
  return num * arguments.callee(num-1);
}
console.log(factorial1(6));//720

let trueFactorial1 = factorial1;
factorial1 = function(){ return 0};
console.log(trueFactorial1(6));

// 720
// 0
// 720
// 720